SMC1A is a member of the cohesion complex that is involved in critical cellular functions such as DNA repair, cell cycle progression, gene expression regulation and the maintenance of genome stability [
11,
26]. Additionally, dysfunction of the proteins in the cohesion complex was found to lead to genome instability, which is directly related to cancer development. In the present study, we found that SMC1A expression was significantly elevated in human CRC tissues. In addition, the increased expression of SMC1A was positively associated with worse clinicopathologic variables, including distant metastasis and higher TNM stage. Interestingly, the results of the current study demonstrated a significant correlation between SMC1A overexpression and shortened OS in patients with advanced CRC. Metastasis and a higher TNM stage are widely believed to be responsible for the worse prognosis noted among patients with CRC. These results suggest that patients with CRC with a higher expression of SMC1A in their tumors have a worse OS.
Previous studies have reported that RNAi-mediated gene silencing could be effectively used to suppress the proliferation of colon cancer cells in vitro [
7,
27]. In the present study, the CRC cell models and mouse models were shown to express elevated amounts of SMC1A. These findings suggested that the overexpression of SMC1A resulted in genome instability and led to the development and progression of CRC cells. shRNA-mediated SMC1A knockdown resulted in a significant down-regulation of cell proliferation, colony formation, cell cycle progression and a significant up-regulation of apoptosis in CRC cells. Our results were coincident with the outcome of SMC1A silencing in lung adenocarcinoma cells, in which cell growth was suppressed through G0/G1 phase cell cycle arrest and apoptosis [
15]. Also, knocking down SMC1A inhibited growth and led to G2/M arrest in glioma cells [
17]. From these data, we suggest that SMC1A plays an essential role in CRC cell growth. Moreover, the size of colon tumors was significantly decreased in the absence of the SMC1A gene, indicating that the up-regulated expression of SMC1A has a positive impact on the progression of CRC cells and that shRNA-mediated gene silencing effectively down-regulates CRC progression in both in vitro and in vivo models.
Intracellular signaling array showed that SMC1A depletion inhibited the activation of Akt and induced the activation of PRAS40. Phosphorylation of Akt at Ser473 and Thr308 by the TORC2 complex and PDK1, respectively, are reliable predictors of Akt activation. Phosphorylation of PRAS40 at Thr246 by Akt relieves the PRAS40 inhibition of TORC1 [
28]. PRAS40 knockdown reduced the ability of tumor necrosis factor (TNF)-α and cycloheximide to induce apoptosis in HeLa cells [
29]. Further analysis demonstrated that SMC1A silencing attenuates the activation of Erk1/2 and Akt, which are generally activated in response to growth factor stimulation that transmits growth and survival signals [
30]. Thus, the mechanisms of SMC1A knockdown restricting CRC cell growth may occur, in part, via the blockade of Akt and MAP kinase activation. Cyclins and cdks are two types of crucial regulatory molecules that determine cell cycle progression [
31]. Cyclin D1 binding to CDK4/6 forms the active complex of Cyclin D1-CDK4/6, which phosphorylates retinoblastoma protein (pRb) and subsequently releases E2F transcription factors, resulting in the activation of specific gene expression required for G1 to S phase progression [
19]. Moreover, SMC1A was suggested to regulate CDK4 as analyzed by SignaLink 2 [
32,
33]. Cell cycle analysis showed that SMC1A knockdown restricted G1 to S phase progression, and further investigation demonstrated that SMC1A silencing down-regulated the expression of CDK4. PCNA is a member of the DNA sliding clamp family of proteins that assists in DNA replication and repair, and considered to be a marker of cell proliferation in various cancers [
34]. In addition, PCNA also forms complexes with Cyclin-CDK complexes and acts as a connector between CDK and its substrates, stimulating their phosphorylation and thus controlling cell cycle progression [
35,
36]. Poly-ADP-ribose polymerase (PARP), a member of the PARP enzyme family, is an abundant DNA-binding enzyme that detects and signals DNA strand breaks [
37]. The presence of cleaved PARP-1 is one of the most used diagnostic tools for the detection of apoptosis in many cell types [
38]. Our results revealed that SMC1A silencing also down-regulates PCNA and PARP expression. Collectively, the mechanisms of SMC1A knockdown inhibiting CRC cell proliferation and cell cycle progression may occur, in part, via the blockade of Akt and MAP kinase activation and the subsequent suppression of CDK4 and PCNA. Knockdown of SMC1A induced apoptosis, potentially due to the induction of PRAS40 and the cleavage of PARP.